The present invention relates generally to wiring pattern inspection apparatus for printed circuit boards, and more particularly to such a wiring pattern inspection apparatus which detects defects of a wiring pattern on a printed circuit board on the basis of a hi-level image produced from an optical grey level image corresponding to the wiring pattern.
In response to the recent requirement for increasing the mounting density of electronic parts on a printed circuit board, improvement for fining a wiring pattern is being made increasingly. On the other hand., for the fining of the wiring pattern, it is required to accurately inspect the abnormality of the wiring pattern. However, difficulty is encountered to keep a high inspection accuracy by the conventional human-eye-based inspection systems and hence it is strongly desired to automatically effect the wiring pattern inspection. One known approach is a defect detecting system such as is disclosed in "Machine vision techniques for inspection of printed wiring boards and thick-film circuits", J. Opt. Amer., Vol. 3, no. 9, pp. 1465-1482, Sept. 1986, written by J. L. C. Sanz and A. K. Jain. The conventional wiring pattern inspection techniques are generally classified mainly into the so-called design-rule system and comparison system. One promising system of these conventional wiring pattern inspection techniques is arranged such that a defect of a wiring pattern is detected by contracting or expanding bi-level image data before the thinning process as disclosed in U.S. Pat. No. 4,853,967 and the document "Novel Method for analysis of printed circuit images" IBM J. Res. Develop., Vol. 29, no. 1, pp. 73-86, Jan. 1985, written by J. R. Mandeville. This system will briefly be described hereinbelow with reference to FIG. 1 in which (A) to (D) are for describing the procedure of the disconnection detecting process and (E) to (F) are for describing the procedure of the short detecting process. In (A) of FIG. 1 showing a defective image, character a represents a non-defective point, and b and c respectively denote fatally defective points accompanying the possibility of line-width abnormality or disconnection. As illustrated in (B) of FIG. 1, a contraction process (erosion process) of the FIG. 1(A) image is first effected so that a defect appears at the point b as disconnection, then followed by thinning the contracted image up to one pixel width as illustrated in FIG. 1(C). Finally, a decision process of the connectivity of the thinned image is effected by scanning 3.times.3 logical masks (indicated by square boxes) and referring to look-up tables as illustrated in FIG. 1(D), thereby detecting the disconnections of the points b and c.
FIG. 1(E) illustrates a defective image where points b and c similarly are fatally defective portions accompanying the possibility of the line-width abnormality or short and a point a designates a non-defective portion. As illustrated in (F) of FIG. 1, an expansion process is first effected by a predetermined size so as to generate a new connected state at the point b, then followed by the thinning process to thin the image up to one-pixel width as illustrated in (G) of FIG. 1, thereafter followed by the 3.times.3 logical scanning process to thereby decide the shorts at the points b and c.
There is a problem with arises with such a conventional wiring pattern inspection system, however, in that, in the case that a micro pinhole which can be disregarded in the inspection is on the wiring pattern, there is the possibility that the line-width abnormality (branching abnormality) of the wiring pattern is excessively detected due to the pinhole. In addition, if a micro conductive portion which can be disregarded in the inspection remains on the base section (the background of the wiring pattern) of the printed circuit board, there is also the possibility that the micro conductive portion is excessively detected as the line-width abnormality (disconnection abnormality).